1. Field of the Invention
The present invention relates to a method for defining the amplitude ratio of a recording mark to be recorded on an information recording medium such as an optical disc or the like and a space. In particular, the present invention relates to a method for adaptably changing the defining technique in accordance with the recording linear density so as to control the recording quality of the recording medium to be within a prescribed range and thus to easily make an optical disc apparatus compatible for recording and reproduction.
2. Description of the Related Art
For example, for an optical disc drive (optical disc apparatus) for recording information on write once optical discs (CD-R, DVD-R, BD-R) or recordable optical discs (CD-RW, DVD-RW, BD-RE) using laser light, it is the most important issue to guarantee the recording quality of information recorded on the optical disc. However, the recording quality of the information recorded on an optical disc depends on various conditions including the using environment of the drive and the temperature when the information is recorded, as well as various conditions including the type of the optical disc and variance among individual optical disc products. Thus, unless information is written by laser light of a recording power suitable to the various conditions at the time of information recording, the information cannot be correctly written. This results in malfunctioning such that, for example, the information cannot be read, or the information which is read is not correct.
In order to stabilize the recording quality of the information recorded on an optical disc, an optical disc apparatus finds the optimum recording laser power for information recording before recording the information on the optical disc. For example, there is a system of finding the optimum recording laser power for information recording by calibration (Optimum Power Control; referred to simply as “OPC”). OPC is performed as follows. Prescribed information is recorded in a power calibration area (referred to simply as “PCA”) of an optical disc while the laser power is changed to a plurality of steps, and the information recorded at each step is reproduced. The maximum value (peak value) A1 and the minimum value (bottom value) A2 of the amplitude of the resultant RF signal with respect to a reference level C are detected. Based on the maximum value A1 and the minimum value A2, a β value is found by the following expression (1).β=(A1+A2)/(A1−A2)  (1)
The laser power at which the obtained β value is a prescribed value is set as the optimum recording laser power for recording, and the information is recorded in the recording area of the optical disc at the optimum recording laser power (see, for example, Patent Document 1 (Japanese Patent No. 3259642)).
The OPC is also performed as follows. Prescribed information is recorded in the power calibration area of an optical disc while the laser power is changed to a plurality of steps, and the information recorded at each step is reproduced. The maximum value (peak value) B1 and the minimum value (bottom value) B2 of the resultant RF signal with respect to a reference level D are detected. Based on the maximum value B1 and the minimum value B2, the modulation degree value is found by the following expression (2).Modulation degree value=(B1−B2)/B2  (2)
The laser power at which the obtained modulation degree value is a prescribed value is set as the optimum recording laser power for recording, and the information is recorded in the recording area of the optical disc at the optimum recording laser power (see, for example, Patent Document 2 (Japanese Laid-Open Patent Publication No. 2006-147125)).
FIG. 17 shows the relationship between an RF signal 110 used for finding the β value and the modulation degree value, and the above-mentioned parameters. The reference level C used for finding the β value is an average level of all the amplitudes of the RF signal, and is an amplitude level equal to the level of a signal which has passed an HPF which is set to pass a prescribed band of signals. Based on the reference level C, the maximum value A1 and the minimum value A2 of the amplitude of the RF signal are found, and the β value is found by expression (1) above.
The reference level D used for finding the modulation degree value is a DC level acting as the reference based on which the amplitude of the RF signal is measured. The reference level D is, for example, an amplitude level equal to the level of a signal when the laser light is extinct. Based on the reference level D, the maximum value B1 and the minimum value B2 of the amplitude of the RF signal are found, and the modulation degree value is found by the expression (2) above.
Conventionally, by using a signal measurement index such as the β value or the modulation degree value to find the optimum recording laser power, the deterioration of the recording quality of information recorded on the optical disc is prevented.
The recording linear density will be described with reference to FIG. 19 and FIG. 20 with a specific example of a BD. Like in a DVD, in the BD also, the recording data is recorded as marks formed by a physical change on the optical disc. A mark having the shortest length among these marks is the “shortest mark 132”. In the case of the BD having a recording capacity of 25 GB, the physical length of the shortest mark is 0.149 μm. This corresponds to about 1/2.7 of that of a DVD. Even if the resolving power of the laser light is raised by changing the parameters of the wavelength (405 nm) and the NA (0.85) of the optical system, the physical length of the shortest mark is close to the limit of the optical resolving power, i.e., the limit at which a light beam can identify a recording mark. FIG. 19 shows how a mark recorded on a track 131 is irradiated with a light beam. In the BD, an optical spot 133 has a diameter of about 0.39 μm because of the above-mentioned parameters of the optical system. When the recording linear density is raised without changing the structure of the optical system, the recording mark becomes small with respect to the diameter of the optical spot, and therefore the resolving power for reproduction is declined.
An amplitude of a reproduction signal obtained by reproducing a recording mark using a light beam decreases as the recording mark is shortened, and becomes almost zero at the limit of the optical resolving power. The inverse of the cycle of the recording mark is called “spatial frequency”, and the relationship between the spatial frequency and the signal amplitude is called OTF (Optical Transfer Function). The signal amplitude decreases almost linearly as the spatial frequency increases. The critical frequency for reproduction at which the signal amplitude becomes zero is called “OTF cutoff”.
FIG. 20 shows the relationship between the OTF and the shortest recording mark regarding the BD having a recording capacity of 25 GB. The spatial frequency of the shortest recording mark of the BD is about 80% with respect to the OTF cutoff, which is close to the OTF cutoff. It is also seen that the amplitude of the reproduction signal of the shortest mark is very small at about 10%. For the BD, the recording capacity at which the spatial frequency of the shortest recording mark is the OTF cutoff, i.e., the recording capacity at which the reproduction amplitude of the shortest mark is almost zero, is about 31 GB. When the frequency of the reproduction signal of the shortest mark is around, or exceeds, the OTF cutoff frequency, the resolving power of the laser light is close to the limit or may exceed the limit. In such an area, the amplitude of the reproduction signal decreases and the S/N ratio is drastically deteriorated.
Now, as indices for defining the recording quality, an asymmetry evaluation index and a P evaluation index will be described.
FIG. 18 shows amplitude levels of a reproduction signal obtained from the longest mark/space (8T) and a reproduction signal obtained from the shortest mark/space (2T). This figure shows a definition of parameters used for calculating the asymmetry evaluation index value. In the case of a medium in which the reflectance of a recorded area is lower than the reflectance of an unrecorded area, the reproduction signal amplitude level of an 8T space is defined as amplitude A8H from reference level 0, the reproduction signal amplitude level of an 8T mark is defined as amplitude A8L from reference level 0, the reproduction signal amplitude level of a 2T space is defined as amplitude A2H from reference level 0, and the reproduction signal amplitude level of a 2T mark is defined as amplitude A2L from reference level 0. The asymmetry evaluation index value (ASYM) is found by the following expression (3).
                    ASYM        =                                                                              A                  ⁢                                                                          ⁢                  8                  ⁢                                                                          ⁢                  H                                +                                  A                  ⁢                                                                          ⁢                  8                  ⁢                                                                          ⁢                  L                                            2                        -                                                            A                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  H                                +                                  A                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  L                                            2                                                          A              ⁢                                                          ⁢              8              ⁢                                                          ⁢              H                        -                          A              ⁢                                                          ⁢              8              ⁢                                                          ⁢              L                                                          (        3        )            
With this index value, it can be defined how much the center of the amplitude of the reproduction signal of the shortest mark/space (2T) or the center of the amplitude of the reproduction signal of the longest mark/space (8T) is DC-shifted with respect to the entire amplitude of the reproduction signal of the longest mark/space (8T). In order to maintain the recording quality to a prescribed level or higher, the recording is controlled such that the asymmetry evaluation index value (ASYM) is within a prescribed range. For example, the asymmetry evaluation index value is restricted to −0.1≦ASYM≦0.1.
However, where the recording linear density is raised and so the frequency of the shortest mark is around, or exceeds, the OTF cutoff frequency, the shortest mark exceeds the optical resolving power. As a result, the amplitude of the reproduction signal is decreased, or does not exceed the reference level due to the influence of inter-code interference. For these reasons, the asymmetry evaluation index value cannot be appropriately found in some cases in an area having a recording linear density exceeding a prescribed level. Namely, in some cases, with the asymmetry evaluation index value, the recording quality cannot be appropriately defined, or the recording quality of the recording medium cannot be controlled to be within a prescribed range. As a result, the recording/reproduction compatibility of the optical disc apparatus cannot be stably maintained.